Paints can be applied to a substrate in a number of ways, e.g., by dipping, brushing, or spraying. For painting large surfaces rapidly, however, the most efficient method for applying paint is by spraying. In automobile production, an automatic paint spraying apparatus is generally employed to paint car bodies on the assembly line. This apparatus includes a color change means which allows the apparatus to spray more than one color of paint. When changing from spraying one paint color to another, the paint flow in the apparatus is stopped and the excess paint in the apparatus is driven out. Afterwards, the spray apparatus is flushed with solvent and air. It is then ready to be used to spray a different color of paint. Many automotive paint spraying assembly line operations change from one color of paint to another every minute, and the cumulative waste of paint that is flushed out of the apparatus over a period of time can be quite costly. It would be advantageous if only a slight excess of paint, over that required to paint the object, would be left in the paint spraying apparatus between color changes. Then, only a small amount of paint would be wasted during the paint cleanout process. This could be accomplished by shutting off the paint prior to the end of the painting cycle and using air or solvent to push the remaining paint through the apparatus to within, for example, a foot of the spray gun. In this way, almost all of the paint in the apparatus could be used in spraying the substrate, and only a small amount of paint would be left in the apparatus between color changes. Consequently, only a small amount of paint, would need to be discharged and disposed of. While both air and solvent can be employed to push the paint out between color changes, their use results in an increased rate of flow of the paint due to a decrease in the friction of the fluid within the paint line. One might attempt to employ metering pumps to control the flow of the paint as it is pushed out by, e.g., solvent. However, currently available gear type, metering pumps are not able to control the speed at which the solvent pushes the paint through the spray apparatus. The difference in viscosity between the paint and the less viscous solvent presents a problem to currently available flushable, metering pumps. These pumps are able to precisely meter materials whose viscosity is similar to that of the paint materials, but are not able to control the metering of the much less viscous solvent to the degree necessary in such painting operations. Since the solvent and paint presents a varied pressure to the pump as solvent travels through the pump, the pump is susceptible to slip. This is particularly a problem in spraying two component paints. In two component paints, the pigmented resin and clear crosslinking agent are fed separately into the spray gun, i.e., they are only combined as they are being sprayed on the substrate. This is in contrast to one component paints which are fed into the spray gun already mixed, i.e., as one component. In two component paint systems, it is critically necessary to maintain a particular ratio of pigmented resin to crosslinking agent in the coating composition. Thus the flow of the two components into the spray gun is carefully metered in a prescribed ratio by means of metering pumps and an electronic control system. If, a solvent were to be used to push the resin out of the apparatus between color changes, the solvent would need to be driven through the apparatus at the speed necessary to maintain the proper ratio of pigmented resin to crosslinking agent in the sprayed composition. If for example, the pigmented resin were to be pushed too fast through the apparatus by solvent, the amount of pigmented resin in the sprayed composition would be more than that necessray for the paint composition. Additionally, the paint could be forced out of the spray gun before the automobile was completely painted, with the result of that solvent would be sprayed on the automobile. Conversely, if the pigmented resin was being pushed by the solvent through the apparatus at too slow a rate, the amount of pigmented resin in the sprayed composition would be less then that necessary for the paint composition, and more pigmented resin would be left in the apparatus between color changes resulting in an excess of paint again being wasted during the flushing. What is necessary is a method for pushing the pigmented resin through the painting apparatus by solvent at a precise rate in order that it may be properly mixed with the crosslinking agent in the spraying apparatus. Additionally, by pushing the resin through the spray apparatus at a precise rate, the resin can be pushed so as to be within a given distance of the spray gun at the time that the substrate has been fully painted and thus minimize the amount of paint which will be flushed out. It is not, however, possible to control with any accuracy the speed at which the pigmented resin would be pushed through the system by solvent if one uses currently available flushable metering pumps to meter the solvent.